1. Field of the Invention
The present invention relates to an optical fiber which is suitable as an optical transmission line in wavelength division multiplexing (WDM) transmission systems.
2. Related Background Art
WDM transmission lines which enable optical transmissions, those of a large capacity and high speed in particular, mainly utilize optical fibers. Recently, however, the deterioration in light signals caused by nonlinear optical phenomena such as four-wave mixing among individual light signals occurring in an optical fiber has become problematic in such WDM transmission systems. Therefore, in the WDM transmission systems, it is important that the occurrence of nonlinear optical phenomena be suppressed, and to this aim, it is necessary that the mode field diameter or effective area of the optical fiber be increased, so as to lower the optical energy density per unit cross-sectional area. For example, Japanese Patent Application Laid-Open No. HEI 8-248251 discloses an optical fiber having an effective area (70 xcexcm2 or more) which is greater than that of normal dispersion-shifted fibers.
It has been known that, in general, microbend characteristics deteriorate as the mode field diameter or the effective area increases, whereby the microbend loss caused by cabling becomes greater.
For example, FIG. 1 is a chart showing the refractive index profile of a dispersion-shifted fiber having a double core structure. In this dispersion-shifted fiber, the core region is constituted by an inner core having a refractive index n1 and an outer core having a refractive index n2 ( less than n1), whereas a single cladding layer having a refractive index n3 ( less than n2) is provided on the outer periphery of the core region. On the other hand, FIG. 2 is a graph showing the relationship between the mode field diameter and the increase in loss caused by microbend at a wavelength of 1.55 xcexcm (1550 nm) concerning this optical fiber having the refractive index profile of a double core structure. In this specification, the mode field diameter refers to Petermann-I mode field diameter. This Petermann-I mode field diameter is given by the following expressions (1a) and (1b):
MFD1=2xc2x7w1xe2x80x83xe2x80x83(1a)
                              w          1          2                =                  2          ·                                                    ∫                0                ∞                            ⁢                                                                    E                    2                                    ·                                      r                    3                                                  ⁢                                  ⅆ                  r                                                                                    ∫                0                ∞                            ⁢                                                E                  ·                  r                                ⁢                                  ⅆ                  r                                                                                        (1b)            
as shown in E. G. Neumann, xe2x80x9cSingle-Mode Fibers,xe2x80x9d pp. 225, 1988.
In expression (1b), r is the radial positional variable from the core center taken as the origin, whereas E is the electric field amplitude and is a function of the positional variable r. The microbend loss is defined by the amount of increase in loss when an optical fiber having a length of 250 m is wound at a tension of 100 g around a bobbin having a barrel diameter of 280 mm whose surface is wrapped with a JIS #1000 sandpaper sheet.
Also, from the results of theoretical studies, it has been known that the relationships of the following expressions (2a) to (2c) exist between microbend loss xcex94 xcex1 and mode field diameter MFD1:                     Δα        =                              1            4                    ·                      (                          1                              R                2                                      )                    ·                                    (                              k                ·                n1                ·                                  w                  1                                            )                        2                    ·                      Φ            ⁡                          (              Δβ              )                                                          (2a)                                Δβ        =                  1                                    w              1              2                        ·            k            ·                          n              1                                                          (2b)                                          Φ          ⁡                      (            Δβ            )                          =                              π                          1              /              2                                ·          Lc          ·                      exp            ⁡                          [                              -                                                      (                                                                  Δβ                        ·                        Lc                                            2                                        )                                    2                                            ]                                                          (2c)            
In these expressions, R is the radius of curvature of microbending applied to the optical fiber, k is the wave number, n1 is the refractive index of the core region, and Lc is the correlation length of the microbending applied to the optical fiber.
As can be seen from FIG. 2 and expressions (2a) to (2c) mentioned above, the microbend loss increases as the mode field diameter MFD1 is greater. However, though the conventional optical fibers are designed in view of macrobend loss, no consideration has been given to microbend loss. Also, it has been known that, if the amount of increase in loss measured when an optical fiber is wound around a bobbin whose surface is wrapped with sandpaper, as an index for cabling an optical fiber, exceeds about 1 dB/km, then microbend loss increases upon cabling. Hence, it is clear that microbend loss increases upon cabling in an optical fiber such as the one mentioned above.
In order to overcome such problems, it is an object of the present invention to provide an optical fiber having, at least, a structure which can effectively suppress the increase in microbend loss.
For achieving the above-mentioned object, the optical fiber according to the present invention comprises a core region extending along a predetermined axis and a cladding region provided on the outer periphery of the core region, these core and cladding regions being constituted by at least three layers of glass regions having respective refractive indices different from each other. Also, this optical fiber is substantially insured its single mode with respect to light at a wavelength in use, e.g., in a 1.55-xcexcm wavelength band (1500 nm to 1600 nm), and has a fiber diameter of 140 xcexcm or more about 200 xcexcm or less. Thus, since the fiber diameter is 140 xcexcm or more, the rigidity of the optical fiber according to the present invention is high even when the mode field diameter is large, whereby the increase in microbend loss is suppressed. On the other hand, since the fiber diameter is not greater than 200 xcexcm, the probability of the optical fiber breaking due to bending stresses is practically unproblematic.
In particular, when the 1.55-xcexcm wavelength band is employed as the wavelength band in use for WDM transmissions, it is preferred in the optical fiber according to the present invention that the absolute value of chromatic dispersion at a wavelength of 1550 nm be 5 ps/nm/km or less. Also, it is preferred that the Petermann-I mode field diameter be 11 xcexcm or more. It is because of the fact that, if the mode field diameter is 11 xcexcm or more, then the optical energy density per unit cross-sectional becomes smaller even when WDM signals are transmitted, whereby the occurrence of nonlinear optical phenomena can effectively be suppressed.
The optical fiber according to the present invention can be employed as a single-mode optical fiber such as dispersion-shifted fiber, dispersion-flattened fiber, dispersion-compensating fiber, or the like.
In particular, when the optical fiber according to the present invention is employed as a dispersion-flattened fiber, it is preferable for the optical fiber to have, for at least one wavelength within the wavelength band in use, a dispersion slope of 0.02 ps/nm2/km or less and an effective area of 50 xcexcm2 or more. More preferably, in particular, the dispersion slope is 0.02 ps/nm2/km or less in terms of absolute value.
Also, when the optical fiber according to the present invention is employed as a dispersion-compensating fiber, it is preferable for the optical fiber to have, for at least one wavelength within the wavelength band in use, a chromatic dispersion of xe2x88x9218 ps/nm/km or less and an effective area of 17 xcexcm2 or more.
Further, when the optical fiber according to the present invention is employed as an optical fiber having an enlarged effective area, it is preferable for the optical fiber to have, for at least one wavelength within the wavelength band in use, an effective area of 110 xcexcm2 or more. The optical energy density per unit cross-sectional area can be kept low in this optical fiber as well, whereby the occurrence of nonlinear optical phenomena can be suppressed effectively.
In various kinds of optical fibers mentioned above, the fiber diameter is 150 xcexcm or more but 200 xcexcm or less. In the case of a dispersion-compensating fiber having such characteristics as those mentioned above, however, its fiber diameter is preferably 140 xcexcm or more but 200 xcexcm or less since its microbend characteristics are likely to deteriorate in particular.
When the optical fiber according to the present invention is employed in an optical cable, it is preferable for the optical fiber to have, for at least one wavelength within the wavelength band in use, an effective area of 17 xcexcm2 or more and a chromatic dispersion value of xe2x88x9283 ps/nm/km or more, and have a fiber diameter of 140 xcexcm or more but 200 xcexcm or less. Such an optical fiber aimed at cabling can be employed as a single-mode optical fiber such as dispersion-shifted fiber, dispersion-flattened fiber, dispersion-compensating fiber, or the like as well.
As explained in the foregoing, in view of various circumstances applicable thereto, the optical fiber according to the present invention is preferably an optical fiber which has a fiber diameter of 140 xcexcm or more but 200 xcexcm or less, and also has, for at least one wavelength within the wavelength band in use, an effective area of 17 xcexcm2 or more and a chromatic dispersion value of xe2x88x9283 ps/nm/km or more; and, further, preferably is an optical fiber which has a fiber diameter of 140 xcexcm or more but 200 xcexcm or less, and also has, for at least one wavelength within the wavelength band in use, an effective area of 17 xcexcm2 or more and a chromatic dispersion value of xe2x88x9248 ps/nm/km or more. Also, depending on the kind of optical fiber employed, the fiber diameter thereof is preferably 150 xcexcm or more but 200 xcexcm or less.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given byway of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.